Apparatus for the production of metals by electrolysis

ABSTRACT

An apparatus for extraction of metals by electrolysis comprising a means for applying a cathodic current of sufficient magnitude with respect to the cross-sectional dimension of an opening in an insulating coating on a cathode so that a powder-like deposit of anode metal will be made on exposed cathode metal, said deposited metal acting to decrease the current density so that subsequent deposited metal will be in a non-powder like consistency.

The present invention concerns a method for the production of metals byelectrolysis from an aqueous electrolyte using at least one anode and atleast one rotational cathode.

BACKGROUND OF THE INVENTION

The use of rotational plate cathodes is described in U.S. Pat. No.1,073,868. The desired metal, here, precipitates onto the cathodes inthe shape of a plate-like coating.

There has not been much practical use of rotational electrodesstationary plate cathodes being mainly in use to day.

The advantage of stationary plate cathodes lies in the simplicity ofoperation and relatively low maintenance costs. They are, however, quitedependent on manual handling in the tankhouse.

The first rotational cathodes, like the stationary plate cathodes,produced platelike cathodic deposits. The only difference was thegeometry of the cathodes. The first mentioned were circular and the lastmentioned rectangular. One of the reasons why rotational plate cathodeswere not widely accepted may be the difficulties experienced instripping the deposited metal from the cathodic material.

Development of the art of chemical processes during later years led tocomplete automation of all unit operations in an integrated process. Inthe case of electrolysis with stationary plate cathodes, partialautomation is achieved by use of computors. The computors keep track ofretention times of the cathodes in the electrolyte, and when theexpected amount of metal is deposited, the computer will send anoverhead crane to pick up the cathodes and move them to the strippingsection. Then, the crane returns with a fresh mother plate cathode tothe vacant place in the electrolytic tank.

Practical operation of such an automated electrolytic process is verycomplicated and many producers, thus, maintain old routines with manuallabour operation.

In order to fully automate an electrolytic process, the concept ofelectrolysis must be changed to a new method maintaining the same metalquality as that obtained by the old methods, at the same costs, butpermitting automation.

SUMMARY OF THE INVENTION

The present invention concerns a method that can be operatedsubstantially continuously and automatic. This is achieved by use of atleast one plate-shaped rotational cathode that is coated with anelectrically insulating coat through which a number of electricalconductors are mounted. Each conductor serves as an area for depositionof the metal. Alternatively, the areas may be small holes made in theinsulating coating.

When said areas are in the shape of holes in the insulating coating, itis a practical advantage to make said holes along a helical path with amutual distance between holes of 0 to 5 mm. When this distance is 0 mm acontinuous helical groove is made on the cathode. The deposited metalcan, then, be withdrawn as a wire. If it is desirable to producecathodes having such a helical groove, said groove may be cut using asharp instrument that will cut through the insulating coating and exposethe underlying elecroconductive core to the electrolyte.

DESCRIPTION OF THE PRIOR ART

As previously mentioned, an apparatus for electrolysis using rotationalcathodes is known from U.S. Pat. No. 1,073,868. According to said patentthe metal was deposited as a continuous coat onto the cathodes, and whenthe pre-set thickness was obtained said coat was stripped. This is anexpensive and complicated process.

Furthermore, according to U.S. Pat. No. 3,860,509 an electrolytic cellis mounted inside a housing and comprises a flat rotational cathodespaced at a short distance from the corresponding anode. The showncathode consists of a number of small diameter cathodic elementsseparated by an insulating matrix. Each element ends in a small tip ontowhich the metal may be deposited as a dendrite that can be scraped offusing a mechanical device mounted on the facing anode surface. Thescraper can be moved in a radial direction and the deposited dendriteson the cathode can, thus, be scraped off from said cathode and may sinkto the bottom to be washed out together with the spent electrolyte whenthe latter is replaced by a fresh electrolyte. The dendrites are thenseparated from the electrolyte by a suitable method.

In U.S. Pat. No. 4,082,641 dealing with stationary plate cathodescomprising a number of electrical conductors separated by an insulatingmaterial, the electrolytic cell mentioned in U.S. Pat. No. 3,860,509 isdiscussed as follows: "This basic concept has been described in U.S.Pat. No. 3,860,509 where it has been used to generate fine, powder-likemetals continuously on microscopic islands, but the technique disclosedtherein is unsuitable for batch converser application where much largerdeposits are involved". As mentioned here, the electrolysis cell of U.S.Pat. No. 3,860,509 is not suitable for industrial use. Additionally, theshown cell is too complicated for practical use.

In U.S. Pat. No. 4,025,400 a continuous process using stationarycathodes is disclosed, where the deposited metal is removed by use of"windscreen wiper"-like devices. The removed metal sinks down throughthe electrolyte onto a conveyor belt which transports the metal out ofthe cell. Such a method, as explained in the last mentioned U.S. patent,is relatively complex as a result of the use of mechanical scrapers usedin a cell having a large number of alternating anodes and cathodes.Another complicating factor is the conveyor belt transporting the metalout of the cell.

According to the present method at least one rotating cathode is used.It is, advantageously, a circular plate. The cathodic material can, e.g.be of the kind described in U.S. Pat. No. 4,193,434, or it may be ametallic material onto which a non-conductive material is nailed in sucha manner that a large number of nails/spikes having a diameter of up to25 mm form the active cathode surface. Such a cathode can bemanufactured in accordance with the method disclosed in the Norwegianpatent application No. 85 0133 (Jan. 11, 1985).

Instead of producing the cathode in accordance with said NO patentapplication No. 85 0133, a cathode may be used where the precipitatedmetal is deposited in holes drilled in the insulating material, or in ahelical groove made in the insulating material. A further, but lessattractive, form of a groove is one extending radially towards theperiphery. Generally speaking, the utilized cathode will comprise anumber of electroconductive areas separated by an electricallyinsulating material.

DESCRIPTION OF THE DRAWINGS

The invention is in the following described with reference to thefollowing figures, where

FIG. 1 is a plan view of a cathodic wheel used in accordance with thepresent method,

FIG. 2 is a plan view of another embodiment of cathodic wheel used inaccordance with the present method,

FIG. 3 is an enlarged fragmentary plan view of the cathodic wheel ofFIG. 1,

FIG. 4 is an enlarged fragmentary plan view of the cathodic wheel ofFIG. 2,

FIG. 5 is a perspective view of a part of an electrolytic apparatus,where the cathodic wheel in use is provided with a helical groove,

FIG. 6 shows a similar arrangement to that of FIG. 5, the cathodicwheel, here, being provided with a number of holes drilled along ahelical path,

FIG. 7 is a perspective view of a an electrolytic cell comprising anumber of anodes and cathodes. In the figure, only cathodes having anumber of holes drilled in the electrically insulating coating are shownwith an additional removing device for removing the deposited metaldifferent from that shown in FIG. 5.

FIG. 8 is a section taken along line A--A of FIG. 3, and FIG. 9 is asection taken along line A--A of FIG. 4.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

FIG. 1 shows a cathodic wheel 1 having an insulating coating 2. 3 is anelectroconductive helical groove area. (Only one groove is shown here.)4 is the hole in the wheel for the shaft. This wheel produces wire.

FIG. 2 shows a cathodic wheel 5 having an insulating coating 6. Aplurality of holes 7 are drilled along a helical path 8. 9 is a hole inthe wheel for the shaft. 10 is the insulating portion between each hole.This wheel produces prills.

As shown in FIG. 3 groove 3 is made in the insulating coating 2. Thebottom of the groove is naked metal 11. FIG. 8 is a cross section of thewire made in the groove 3. 12 shows where the first metal is depositedwhich has a "rotten" texture. (8) shows the zone where "brittle" metalis located whereas (9) indicates the zone where solid metal is located.

FIG. 4 shows the helical path along which holes 7 are drilled in theinsulating coating 6, and 15 indicates the conductive metal bottom inthe hole 7.

FIG. 9 shows a section of a prill, where 16 is the "rotten" zone firstdeposited at a very high current density. 17 shows the brittle zone, and18 shows the zone where the solid metal is deposited.

FIG. 5 shows the cathodic wheel of FIG. 1, where the metal is formed inthe helical groove 3. 19 is the wire remover (cropper, harvester)controlled by 120. The wire taken off is wound by 21 and a bundle 22 canbe removed. 23 is the anode, and 24 is the tank with an electrolyte 25.

FIG. 6 shows the cathodic wheel of FIG. 2, where (2) indicates holesdrilled along a helical path, as shown in FIG. 2. 26 designates theprill remover (cropper, harvester) which is controlled by 27. The prillsare sucked by a suction system 28 down into hopper 29 and are dischargedinto conduit 30. 32 is an anode in a tank 32 containing an electrolyte33.

In FIG. 7 a group of rotating plate cathodes such as 5 are arrangedalternately with anodes 34 in a tank 35. Cathode 5 is provided with anumber of electroconductive areas 7 separated by an electricallyinsulating material. Such a cathode, thus, represents one of thepreviously disclosed cathodic materials. The plate cathodes are mountedon a rotating shaft 36.

The anodes and cathodes are connected to (not shown) an external powersupply via current bus-bars 37 and 38 respectively. The electrolyte isadded to the tank 35 through a supply pipe or conduit 40 and spentelectrolyte is removed from tank 35 through a corresponding pipe orconduit 41. The metal deposited on the cathodes is removed by use ofmechanical scraper 42 and the removed metal 43 falls down onto aconveyor 44 and is removed from the system. In the figure only onescraper on one side of cathode 1 is shown, whereas in practice, ofcourse, a scraper on each side of each rotating cathode 1 will be used.

When a helical groove 3 is cut in the cathodic coating it is,preferably, made in such a manner that the width of the conductive metalbottom of the groove is in the range of 0.05-0.2 mm. When holes 7 aredrilled in the insulating coating on the cathode, the metallic bottom ofthe hole, preferably, has a diamter in the range of 0.1-0.5 mm for theproduction of prills.

Persons skilled in the art of electrolysis will know that differentmetals deposited by electrolysis will show varying rigidity andhardness. A hard and brittle metal may, advantageously, be deposited asprills, and a soft metal may, advantageously, be deposited as a wire byusing a cathode with a helical groove cut into it.

The present method will be further described by the following examples.

EXAMPLE 1

The object of this example was to prove that copper prills can be madeby electrolysis in a standard CuSO₄ /H₂ SO₄ electrolyte using a rotatingcathode coated with a plastic coating into which a number of holes hadbeen made, thus, exposing the underlying cathode metal to theelectrolyte through said holes.

Test conditions were as follows:

    ______________________________________                                                       8                                                              Rotation of cathode                                                                          2 rpm                                                          Temperature    40° C.                                                  Anode          Copper                                                         Cathode        Plastic coated stainless stell                                                plate having 200 holes with dia-                                              meter 0,5 mm. Cathode diameter =                                              200 mm.                                                        Current        0,2 amps at start                                                             4,5 amps at the end                                            Cell voltage   0,3 volts                                                      Submersion of cathode in                                                                     45% of total cathodic area.                                    the electrolyte                                                               ______________________________________                                        Table 1 - Results                                                             Time    Average prill weight                                                                         Average prill diam.                                    (hrs)   (mg)           (mm)                                                   ______________________________________                                        17.7    42             2,7                                                    ______________________________________                                    

The test shows that almost perfect semi-spherical prills of copper wereproduced in a size that could easily be stripped off after 17.5 hours ofelectrolysis. The prills were solid and could easily be washed to removetraces of electrolyte.

The electrolytic cell was operated on a constant cell voltage of 0.3volts, thus, varying the current density in accordance with the size ofthe prills produced.

In practice an even current distribution is expected and hence aconstant cell current and voltage, this because several cathodes will beutilized in a cell and only some of the cathode sides will be strippedat any given period of time.

EXAMPLE 2

The object of this example was to show that prills are also formed whenthe diameter of the hole exposed to the electrolyte (hereafter called"island") was larger than 0.5 mm. The diameter was varied from 0.5 to4.5 mm, but the test was carried out as in example 1 for the rest.

                  TABLE 2                                                         ______________________________________                                        Results                                                                            Island                      Theoretical                                       diam.   Average prill                                                                            Average prill                                                                          weight                                       Time (mm)    diam. (mm) weight (mg)                                                                            (mg)     F                                   ______________________________________                                        17.5 0,5     2.7        42 (ex.1)                                                                               44      0.95                                50   1.5     5.0        270      280      0.96                                33   2.5     5.0        260      280      0.93                                80   4.5     8.0        650      1140     0.57                                ______________________________________                                         F = a factor showing the ratio between the weight of the deposited prill      and the weight of a perfect semispherical ball having the same diameter a     the deposited prill.                                                     

The test shows that the prills produced were almost perfectsemi-spherical balls when the island diameter was less than 2.5 mm. Thesemi-spherical prills were easier to strip off than prills made onislands having a diameter of more than 2.5 mm. This indicates that it isadvantageous, in practical operation, to use islands having a diameterof less than 2.5 mm.

EXAMPLE 3

This example was carried out to show the advantage of using rotationalcathodes as compared to stationary plate cathodes. A zinc anode was usedin a zinc chloride electrolyte. The cathode was a rotational aluminiumplate coated with a 2 mm thick plastic plate nailed to the aluminiumcore by use of aluminium nails. It was, in other words, produced inaccordance with NO patent application No. 85 0133. The heads of thenails served as islands, and during electrolysis zinc was deposited onsaid islands. The diameter of said islands was 4.5 mm and thetemperature was 32.5° C. The electrolyte contained 25 g/l Zn⁺⁺ and thepH was adjusted to 2 using HCl. No organic polymers were added.

                  TABLE 3                                                         ______________________________________                                        Results                                                                       Time                Current eff.                                                                            Energy used                                     (hrs)  RPM          (%)       (kwh/ton Zn)                                    ______________________________________                                        24     0            75.2      1210                                            32     1            98.4      600                                             22     2            95.2      630                                             23     6            91.3      670                                             ______________________________________                                    

The zinc prills were flat but easy to strip off from the cathode. Thecurrent was almost constant at 1.0-1.3 amps with a cell voltage of0.6-0.8.

The test clearly indicates that it is advantageous to use rotationalcathodes in the present method, the rotational cathode causing goodstirring of the electrolyte in the tank and, thereby, decreasing oreliminating the diffusional zones along the cathode caused by thehydrogen bubbles, as well as denudation of the electrolyte w.r.t. zincions.

EXAMPLE 4

The object of this test was to produce wire instead of prills of copper.

A circular cathode wheel was made from stainless steel with a diameterof 1.0 meter and was coated with an epoxy resin. On one side, a helicalgroove was cut in the epoxy resin down to the underlying metal in such amanner that the bottom of the groove was a 0.2 mm wide metal band havinga length equal to the entire length of the groove. The helical groovehad a pitch of 5 mm, so that the total length of the spiral was 140meters, starting from the cathode's outside (D=0.98 m) to an innerdiameter of 0.25 meters.

Said wheel was submerged in a standard copper electrolyte to 40% of thetotal cathode surface, and the current flow was started. After 35 hoursof electrolysis at 17 amperes, 610 g of copper-wire were stripped fromthe wheel portion above said electrolyte. This wire had a diameter ofabout 1.0 mm and a cross-section almost perfectrly semi-circular.

    ______________________________________                                        Test data                                                                     ______________________________________                                        Anode        Lead (3% Sb stabilized)                                          Cathode      Stainless steel, epoxy resin coated on                                        both sides.                                                      Electrolyte  Copper sulphate/sulphuric acid                                                (60 g/l Cu, 100 g/l H.sub.2 SO.sub.4)                            Temperature  79° C.                                                    Cell voltage 1.66 V (at the end)                                              ______________________________________                                    

CONCLUSIONS

The initial current density was so high that the bottom of the wire (themetal first deposited in the groove) was "rotten" and appeared as a darkpowder. As the wire grew current density was decreased towards 1.7A/dm². This produced a solid, shining metal wire. Stripping of said wirewas very easy due to the "rotten" core made initially. This method ofelectrolysis is intentional and a preferred method in accordance withthe present invention.

Stripping was performed using a "pick-up" which was provided with asmall stainless steel knife on the end. Said "pick-up" was a hollow tubeconnected to a spooling arrangement. The wire loosened by the knife waseasily transported down the tube to the spooler where a coil was made ofthe wire produced. The "pick-up" easily followed the helically formedwire on the cathode.

EXAMPLE 5

The object of this test was to make nickel prills.

A circular cathode wheel made from stainless steel and having a diameterof 1.0 m was coated with an epoxy resin. On one side 17 500 holes weredrilled in such a manner that the bottom of the holes exposed theunderlying metal core. The diameter of this metallic bottom was 0.2 mm.Said holes were drilled sequencially along a helical path 8 mm apart.The pitch of said path was 5 mm, the total length of said helical path,thus, being 140 m, starting from the cathode outside (D=0.98 m) to aninner diameter of 0.25 m.

    ______________________________________                                        Test data                                                                     ______________________________________                                        Cathode       Stainless steel, epoxy resin coated                                           on both sides                                                   Anode         Ruthenium coated titanium                                       Electrolyte   Nickel sulphate/-chloride                                                     (Ni = 60 g/l, pH = 1.3-1.5)                                     Temperature   77° C.                                                   Cell voltage  2.12 V (at the end)                                             ______________________________________                                    

CONCLUSIONS

After 32 hours of electrolysis at a constant current of 17 amps, 530grams of nickel prills were easily stripped from the cathode wheel.

The initial current density was so high that the bottom of prills (themetal initially deposited in the drilled holes) was "rotten" andconsisted of a dark powder.

As the prills grew current density decreased towards 2.5 A/dm². Thisproduced solid and shining metal prills. Stripping the prills was veryeasy due to the "rotten" core initially formed. This procedure isententional and a preferred method in accordance with this invention,both as regards wire and prills.

Stripping was performed using a "pick-up" provided with a smallstainless steel knife at the end. The "pick-up" was a hollow tubeconnected to a suction system and a cyclone. The prills loosened by theknife were easily and efficiently sucked into said "pick-up" and thendown into the cyclone, from which they were discharged after endedstripping. The "pick-up" easily followed the helical path made by theprills.

This shows that the present invention is flexible encompassing a cathodehaving at least one continuous grove/side to a cathode having its groovedivided into smaller portions (holes) and, thus, producing prillsinstead of wire.

EXAMPLE 6

The object of this test was to produce nickel wire. The electrolyte andthe procedure from example 5 were used, but the cathodic wheel wasreplaced by one as used in example 4.

After ended electrolysis the nickel wire produced was stripped off andspooled to a coil as mentioned in example 4. This shows that the presentinvention is also flexible so as to encompass production of nickel wire.

It was found that the cathode in the pilot plant could be submerged tobetween 30 to 70% of its total surface area into the used electrolyte.

We claim:
 1. An apparatus for extraction of metals by electrolysis,comprising a bath to contain an electrolytic solution, a metalplate-like cathode having a substantially flat surface, an anode formedof a material to be deposited and connected electrically to said cathodein a cathodic circuit, means for rotating said cathode in said bath, aninsulating coating disposed on said surface, said coating having atleast one opening to expose the cathode to said solution, means forapplying a current to the cathodic circuit with said current being ofsufficient magnitude with respect to the cross-sectional dimension ofsaid opening so that a powder-like deposit of said anode metal will bemade on the exposed cathode, said deposited metal acting to decrease thecurrent density so that subsequent deposited metal will be in anonpowder-like consistency.
 2. The apparatus of claim 1, wherein saidopening comprises a multiplicity of spaced holes.
 3. The apparatus ofclaim 1, wherein said opening comprises a helical groove.
 4. Theapparatus of claim 1, wherein the opening is generally V-shaped in crosssection and is bordered by spaced, outwardly diverging walls.
 5. Amethod for the electrodeposition of a metal, comprising the steps ofapplying an insulating coating to a generally flat surface of aplate-like cathode, forming at least one opening in said insulatingcoating, positioning said cathode in an electrolytic bath, connectingthe cathode in a cathodic circuit with an anode of a metal to be plated,rotating said cathode in said bath, applying a current of sufficientmagnitude with respect to the area of said opening to provide an initialcurrent density of a magnitude to form a powder-like deposition of saidanode metal at the exposed portion of said cathode, and thereafterreducing the current density to form an outer deposited portion of saidanode metal in a nonpowder-like consistency.
 6. The method of claim 5,wherein current is applied in a manner such that the deposited metal hasa progressively greater surface area in a direction outward of saidcathode.